mikros/rust
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

borrowck/README.md: Remove most references to &const

Browse Source
This commit is contained in:
Keegan McAllister 2015-02-18 17:59:47 -08:00
parent dfc5c0f1e8
commit bb22c100db
1 changed files with 9 additions and 24 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

33
src/librustc_borrowck/borrowck/README.md
View File

@ -64,7 +64,7 @@ Here is the formal grammar for the types we'll consider:
```text
TY = () | S<'LT...> | Box<TY> | & 'LT MQ TY
MQ = mut | imm | const
MQ = mut | imm
```
Most of these types should be pretty self explanatory. Here `S` is a
@ -198,7 +198,7 @@ The kinds of expressions which in-scope loans can render illegal are:
Now that we hopefully have some kind of intuitive feeling for how the
borrow checker works, let's look a bit more closely now at the precise
conditions that it uses. For simplicity I will ignore const loans.
conditions that it uses.
I will present the rules in a modified form of standard inference
rules, which looks as follows:
@ -275,15 +275,14 @@ but also the code in `mem_categorization`.
Let's begin with the rules for variables, which state that if a
variable is declared as mutable, it may be borrowed any which way, but
otherwise the variable must be borrowed as immutable or const:
otherwise the variable must be borrowed as immutable:
```text
MUTABILITY(X, MQ) // M-Var-Mut
DECL(X) = mut
MUTABILITY(X, MQ) // M-Var-Imm
MUTABILITY(X, imm) // M-Var-Imm
DECL(X) = imm
MQ = imm | const
```
### Checking mutability of owned content
@ -304,12 +303,11 @@ MUTABILITY(*LV, MQ) // M-Deref-Unique
### Checking mutability of immutable pointer types
Immutable pointer types like `&T` can only
be borrowed if MQ is immutable or const:
be borrowed if MQ is immutable:
```text
MUTABILITY(*LV, MQ) // M-Deref-Borrowed-Imm
MUTABILITY(*LV, imm) // M-Deref-Borrowed-Imm
TYPE(LV) = &Ty
MQ == imm | const
```
### Checking mutability of mutable pointer types
@ -466,15 +464,12 @@ are computed based on the kind of borrow:
```text
&mut LV => RESTRICTIONS(LV, LT, MUTATE|CLAIM|FREEZE)
&LV => RESTRICTIONS(LV, LT, MUTATE|CLAIM)
&const LV => RESTRICTIONS(LV, LT, [])
```
The reasoning here is that a mutable borrow must be the only writer,
therefore it prevents other writes (`MUTATE`), mutable borrows
(`CLAIM`), and immutable borrows (`FREEZE`). An immutable borrow
permits other immutable borrows but forbids writes and mutable borrows.
Finally, a const borrow just wants to be sure that the value is not
moved out from under it, so no actions are forbidden.
### Restrictions for loans of a local variable
@ -641,19 +636,6 @@ in terms of capability, the caller passed in the ability to mutate
`*p` is borrowed since that would be a move of `p`, as `&mut` pointers
are affine.)
### Restrictions for loans of const aliasable referents
Freeze pointers are read-only. There may be `&mut` or `&` aliases, and
we can not prevent *anything* but moves in that case. So the
`RESTRICTIONS` function is only defined if `ACTIONS` is the empty set.
Because moves from a `&const` lvalue are never legal, it is not
necessary to add any restrictions at all to the final result.
```text
RESTRICTIONS(*LV, LT, []) = [] // R-Deref-Freeze-Borrowed
TYPE(LV) = &const Ty
```
### Restrictions for loans of mutable borrowed referents
Mutable borrowed pointers are guaranteed to be the only way to mutate
@ -855,6 +837,9 @@ prohibited from both freezes and claims. This would avoid the need to
prevent `const` borrows of the base pointer when the referent is
borrowed.
[ Previous revisions of this document discussed `&const` in more detail.
See the revision history. ]
# Moves and initialization
The borrow checker is also in charge of ensuring that: